def is_isogram(string):
    '''Determine if an input string is an isogram.
    
    Isogram means that the word contain no duplicated letters (case insensitive)'''
    chr_list = list(string.lower())
    alpha_list = []
    for i in chr_list:
        if i.isalpha():
            alpha_list.append(i)
    if len(alpha_list) == len(set(alpha_list)):
        return True
    return False